If you want to change your verified mobile phone number, the following two methods can help you.
Note: The verified phone number is bound to your Futubull account, which can be used to log in and receive account-related notices. If you want to receive trade-related notices, please change the verified phone number for your brokerage account on the account information page.
Futubull app - Me - Settings - Account and Security - Phone Number
Futubull - Settings - Account and Security - Phone Number